1. Field of the Invention
The present invention relates to an apparatus for boring perforations in a web sheet such as a tipping paper sheet, and more particularly, to an apparatus which can bore in a web sheet perforations of a uniform shape at high speed.
2. Description of the Related Art
Various apparatuses are known which bore perforations in a rolled paper sheet or a web sheet such as a tipping paper sheet. In these apparatuses, a pulsating laser beam is used to form a number of perforations having a uniform shape and an accurate pattern in a rolled paper sheet or a web sheet such as a tipping paper sheet. Various methods are available for converting a laser beam into a pulsative one. Among them are: (1) pulse beam oscillation method; (2) slit method; (3) chopper method, (4) shutter method, i.e., beam scanning method, and high-speed polygon mirror method.
In the pulse beam oscillator method, a carbon dioxide gas laser, which generates a pulsating laser beam, is usually employed as a laser beam source. This is because the pulsating laser beam generates a wavelength of 10 .mu.m which can be optimally absorbed into the water contained in a web sheet. At present, the carbon dioxide gas laser is considered to be more suited for boring perforations in paper than any other type of a laser hitherto developed. This laser, however, requires hundreds of microseconds to increase the beam energy from the zero level to the highest level and decrease it back to the zero level. Consequently, the laser cannot generate beam pulses at sufficiently short time intervals. Hence, there are limits to the speed at which a boring apparatus incorporating the carbon dioxide gas laser can bore perforations in a web sheet. More specifically, the maximum speed is no more than 2 mm/sec for perforations arranged at a pitch of 1 mm, and the boring efficiency is relatively low.
In the slit method, a slit member having a row of slits is moved, along with a paper sheet located below the slit member, while a continuous laser beam converged by a lens system is applied to the slit member. As a result, beams are applied through the slits onto the paper sheet, thereby forming perforations in the paper sheet. Since it suffices for the laser to emit a continuous beam, not a pulsative beam as in the pulse beam oscillation method, perforations can be formed in the paper sheet at a higher speed, e.g., 6 mm/sec, and hence, at a lower cost than in the case where the pulse beam method is employed. The slit method, however, is disadvantageous in the following respects:
(a) Since the slits can hardly be formed with so high an accuracy as to have the same size, the perforations formed by using the slit inevitably vary in size and pitch. In this respect the slit method is generally inferior to the pulse beam oscillation method. PA1 (b) The slits are located substantially at the focal point of the lens system, and the beam converged by the lens system is continuously applied to the slit member. The converged beam, which is intense, may damage the slit member, and the slits may thereby vary in size, giving rise to variations in the size of and pitch of perforations. PA1 (c) The laser beam may burn the paper sheet in the process of perforating the sheet, forming ashes on the sheet. The ashes may adhere to the slit member, narrowing the slits and eventually hindering the passage of the beam through each slit. If this happens, the resultant perforations in the paper sheet will vary in size. PA1 a laser beam source for generating a continuous laser beam; PA1 a first converging lens for converging the laser beam generated by the laser beam source; PA1 a rotary polygon mirror for deflecting the laser beam converged by the first converging lens; PA1 a beam splitting/reflecting mirror for splitting the laser beam deflected by the rotary polygon mirror, into segment beams and reflecting the segment beams; PA1 a converging-lens system for converging the segment beams applied from the beam splitting/reflecting mirror, on the surface of a web sheet; and PA1 a second converging lens located between the rotary polygon mirror and the beam splitting/reflecting mirror, for converging the segment beams and applying the converted segment beams at a constant speed to a limited surface region of the web sheet.
In the chopper method, a continuous laser beam is applied to a rotating chopper disk, which chops the beam into segment beams. The segment beams are sequentially applied onto a paper sheet, whereby perforations are formed one after another in the sheet and located at regular intervals. The chopper method is free of the problems inherent in the pulse beam oscillation method and the slit method. However, the chopper disk must be rotated at high speed, and may be broken while rotating at high speed, due to centrifugal force. In the case of a chopper disk having a diameter of 30 cm and capable of chopping the beam 20 times per rotation, it forms only 20,000 segment beams every minute when rotated at 1,000 rpm, and forms as many as 1,200,200 segment beams when rotated at 60,000 rpm. If the disk is rotated at so high a speed as 60,000 rpm, it is quite probably that the disk will be broken.
In the shutter method, a continuous laser beam is directed to a plurality of rotating disks, each having reflective surfaces and openings, as is disclosed in Published Examined Japanese Patent Applications 57-37437, 57-49318, and 59-318. As the beam is reflected by reflective surfaces and output through the holes, it is divided into segment beams. The segment beams are applied onto a paper sheet, thus perforating the sheet. The shutter method can form rows of perforations in the sheet at the same time. This means effective use of laser power, which results in a high-efficiency perforating process. The same number of rotating disks as the number of rows of perforations must be used, however. Moreover, the disks must be made with a very high precision. Otherwise, the disks will fail to rotate at a constant speed, giving rise to a perforation inaccuracy, and will be broken in the worst case. To have many reflective surfaces and many openings, each disk needs to have a large diameter. It would therefore be difficult to machine the disk with a high precision. This may also invite destruction of the disk.
The high-speed polygon mirror method is disclosed in Published Unexamined Japanese Patent Application 3-40191. This method uses a polygon mirror and a plurality of mirrors. A continuous laser beam is applied to the polygon mirror which is rotating at high speed, and deflected thereby, thus the beam is chopped into segment beams. The segment beams are applied to the mirrors, which reflect the segment beams to a plurality of lenses, respectively. The lenses converge the segment beams. The segment beams are sequentially applied onto a paper sheet, whereby the sheet is perforated. Since all laser power generated by the laser is utilized exclusively for perforating the sheet, the energy-efficiency is high in the high-speed polygon mirror method. Further, because the method helps form rows of perforations at the same time, the perforation efficiency can increase. Since the polygon mirror has a smaller diameter than the disks used in the chopper method and the shutter method, it can be made with high precision and can, therefore, be rotated at a high constant speed and eventually serves to perforate the sheet with high precision. More specifically, a polygon mirror having a diameter of about 40 mm, can be rotated at so high a speed as 120,000 rpm so that perforations can be formed in the paper sheet at a pitch of 0.5 mm and at a speed of 24 m/sec.
In the pulse beam oscillation method, the laser emits beams intermittenly, and its beam-emitting efficiency is inevitably low. In the slit method and the shutter method, the continuous laser beam is repeatedly shielded by the slit member or the chopper disk so frequently that its energy is not effectively utilized. In the shutter method, the disks must be rotated at a speed sufficiently high to perforate the paper sheet at high speed, and may be broken when rotated so fast. The problems of these methods are not found with the high-speed polygon mirror method, which can form perforations of a uniform shape in a paper sheet at a sufficiently high speed. The high-speed polygon mirror method is, however, disadvantageous. Since the segment beams generated by deflecting the laser beam by the rotating polygon mirror are reflected by the mirrors and applied to the lenses, the lenses may focus the segment beams at different positions, forming beam spots of different sizes, because they differ in aberration. In other words, the lenses do no have the same focal distance. Consequently, the perforations formed in the paper sheet by applying the segment beams focused by the lenses may have different sizes. In the worst case, some of the lenses fail to converge the input pulses into beams slender enough to form perforations in the sheet, and the sheet will have regions perforated as desired and regions not perforated at all.
As indicated above, the boring apparatuses employing the existing methods of converting a laser beam into a pulsative one cannot perforate a paper sheet at high speed, cannot form perforations of a uniform size, cannot utilize the laser-beam energy efficiently, or may be damaged if the disk or disks used have not been machined with high precision.